Main Vectors of Evolution in Flowering
Plants (The Criteria Used in Evaluating
the Relative Degree of Their
Advancement)
The vegetative characters there are many easily
rever sible characters, such as growth habit, arrange-
ment, size and form of leaves, but there are also many
trends which either can be reversible with great diffi -
culty or are completely irreversible. In general, vegeta-
tive organs are characterized by more reversibility than
reproductive organs. However, even the most revers-
ible characters usually reveal more or less defi nite
evolutionary trends.
Growth habit: The most primitive magnoliophytes
are woody plants, and the herbaceous growth habit is
always secondary (Jeffrey 1899, 1917; Hallier 1905,
1912; Sinnott and Bailey 1914, and many subsequent
authors including Eames 1961, and Stebbins 1974).
The evolution of fl owering plants most probably begins
with small, relatively weakly branched woody forms.
According to Hallier (1912) the early angiosperms
were small trees with a weak crown of relatively few
thick branches, like the fossil bennettitaceous genus
Wielandiella or some living cycads. Stebbins (1974),
on the other hand, visualizes the earliest angiosperms
as low-growing shrubby plants, having a continuous
ring of secondary vascular tissue, and no single well-
developed trunk. Amongst the living primitive fl ower-
ing plants there are both trees (the majority) and shrubs
(Eupomatia laurina, for example, is a shrubby plant
with several trunks). It is diffi cult to say whether the
earliest magnoliophytes were small trees or shrubs. The
only thing we can say is that they were small woody
plants, which occupied only a modest and insignifi -
cant position in the Early Cretaceous vegetation. Big
stately trees of tropical rain forest are derived, having
originated from primitive, small, woody angiosperms.
Trees with numerous slender branches evolved from
sparingly branched trees. Deciduous woody plants
evolved from evergreen ones.
The evolutionary trend from woody plants to herbs
is not irreversible. In some phyletically distant taxa of
fl owering plants the reverse process of the transform-
ation of herbaceous plants into arborescent plants took
place, for example, in Ranunculaceae, Berberidaceae,
Papaveraceae, Phytolaccaceae, Nyctaginaceae, Cheno-
podiaceae, Polygonaceae, Cucurbitaceae, Campa-
nulaceae-Lobelioideae, Asteraceae, and many liliopsids
(including Agavaceae, Dracaenaceae, Philesiaceae,
Smilacaceae, Poaceae – Bambusoideae, Arecaceae,
Pandanaceae). But usually these secondary arbores-
cent plants, especially arborescent liliopsids, strikingly
differ from the primary woody plants. As Stebbins
(1974: 150) aptly remarks, “Palms and bamboos are as
different from primitive preangio- spermous shrubs
and trees as whales and seals are from fi shes”.
Branching: There are two main morphological
types of branching in fl owering plants – monopodial
and sympodial. Both these types are met in many fami-
lies and even within one and the same genus and
change from one to the other with great ease. This
makes the determination of the main direction of evo-
lution of the branching in fl owering plants somewhat
diffi cult. The study of the most archaic extant magno-
liophytes indicates that perhaps the original type has a
combination of monopodial and sympodial branching –
well expressed, for example, in Magnolia. The vegeta-
tive branches of Magnolia are monopodial, but the
short branches carrying the terminal fl owers develop in
a strictly sympodial manner, and the apparently simple
axis of such a branch is in fact a sympode of a certain
number of shoots of an ascending series. The sympodial
Introduction
xiii